As disclosed in U.S. Pat. No. 3,255,272 (Lindsay), it is known that alkenes can be dimerized in the presence of supported or unsupported alkali metals as catalysts, although the patentee appears to indicate that the supported catalysts are inferior to unsupported catalysts in the reactions. Such processes are of interest for the dimerization of alkenes in general, but they are of particular interest for the dimerization of normal 1-alkenes containing 3-8 carbons, especially propene.
The dimerization of propene typically results in the formation of a mixture of hexenes, one of which is 4-methylpentene-1, a compound which is useful as a fuel additive and as a monomer which can be employed to prepare desirable homopolymers and copolymers. The more attractive propene dimerizations are those which maximize the amount of 4-methylpentene-1 formed.
Japanese Published Applications 61-78736 (Nippon Oil) and 61-83135 (Nippon Oil) teach that propene can be dimerized with high selectivity to 4-methylpentene-1 in the presence of a catalyst composition obtained by (1) heating a mixture of appropriate potassium and aluminum compounds at 400.degree.-2000.degree. C. for 1-20 hours to prepare a support corresponding to the formula K.sub.2 O.xAl.sub.2 O.sub.3, (2) supporting at least one of sodium, potassium, sodamide, and potassium amide thereon, (3) optionally treating the product with hydrogen and/or oxygen, and (4) finally treating the product with an ester. However, the preparation of these catalyst compositions is inconvenient.
European Patent Application 83083 (Kawamoto et al.) discloses the dimerization of propene with high selectivity to 4-methylpentene-1 in the presence of a catalyst composition obtained by dispersing metallic sodium and metallic potassium on a molded article comprising an anhydrous inorganic potassium compound and elemental carbon.
It has now been found that improved results can be obtained by heating a normal 1-alkene containing 3-8 carbons in the presence of a supported alkali metal as a catalyst and in the presence of at least one oxide of sodium, potassium, rubidium, or cesium as a co-catalyst, the catalyst composition containing about 200-1500 weight % of support and about 10-100 mol % of co-catalyst, based on the amount of alkali metal catalyst.
Alkenes which may be dimerized by the process of the invention are normal 1-alkenes containing 3-8 carbons, i.e., propene, butene-1, pentene-1, hexene-1, heptene-1, and octene-1. Propene is preferred.
As in Lindsay, the teachings of which are incorporated herein in toto by reference, the alkali metal employed as a catalyst may be lithium, sodium, potassium, rubidium, or cesium. However, it is preferably potassium or a potassium alloy, e.g., NaK. The amount used is a catalytic amount, generally about 2-10 mol %, based on the amount of alkene.
The alkali metal appropriately has its surface area increased by being finely divided or liquid as well as by being supported on any suitable support material, such as diatomaceous earth, activated charcoal, granular coke, silica, alumina, zeolite, pumice, porcelain, quartz, steel turnings, copper shot, sodium carbonate, potassium carbonate, etc.
The supported catalyst is preferably prepared by dispersing the alkali metal onto the support, which may already have the co-catalyst deposited thereon, in the absence of the alkene and any diluent.
The co-catalyst of the invention is an oxide of sodium, potassium, rubidium, and/or cesium and is preferably sodium oxide. Like the alkali metal, it is used in finely-divided form; and it may be incorporated into the reaction mixture as the oxide, or it may be generated in situ, e.g., by oxidizing the supported alkali metal catalyst when the catalyst is sodium, potassium, rubidium, or cesium. Alternatively, as in a preferred embodiment of the invention, the co-catalyst may be incorporated by adsorbing onto the catalyst support the alkali metal corresponding to the desired oxide and then oxidizing the adsorbed metal to the oxide.
The reaction is conducted by heating a mixture of the alkene, the supported catalyst, and the co-catalyst under substantially anhydrous conditions at a suitable temperature, generally about 100.degree.-250.degree. C., preferably about 150.degree.-200.degree. C., to dimerize the alkene. It may be conducted in the absence of a diluent or in the presence of an excess of the alkene as the sole diluent. However, it is usually conducted in an inert diluent, e.g., a liquid alkane, cycloalkane, or aromatic hydrocarbon, such as pentane, hexane, heptane, isooctane, cyclohexane, naphthalene, decahydronaphthalane, white oils, etc.
The process of the invention proceeds at a faster rate and/or provides higher product yields with fewer by-products than comparable processes conducted in the absence of the co-catalyst. It is advantageous as a means of preparing compounds which are useful as solvents, internal standards, polymer intermediates, etc., and is particularly advantageous as a method of preparing 4-methylpentene-1 in a predominant amount.
The extent to which the use of both the support and the co-catalyst increases the selectivity to 4-methylpentene-1 in the dimerization of propene is surprising. Comparison of experiments in which both the support and the co-catalyst were employed with experiments in which neither was used, experiments in which only the support was utilized, and experiments in which only the co-catalyst was utilized demonstrate that the support and co-catalyst act synergistically to provide an increase in selectivity that is greater than the additive effect that might have been expected from the results achieved by the use of the supports and co-catalysts separately.